2 9 6 



THE CEPHALOPODA 



but are reduced so as to be of the same width throughout (Fig. 

 272, P). The fins of Ctenopteryx are similar, but are pectinated, that 

 is to say, they consist of a thin membrane supported by muscular 

 fibres (Fig. 2">9, Ji). On the other hand, in species with a short and 

 globular mantle the fins are shifted away from the aboral extremity, 

 and are situated either in the middle of the body, as in 

 or even near the anterior end, as in Cirrko- 

 teuthis (Fig. 260). 



Except in Nautilus (Fig. 270, /) and 

 Spirula (Fig. 268, pa.c), where it is shallower, 

 the pallial cavity extends from its opening 

 behind the head to the aboral extremity 

 or summit of the body. It contains the 

 branchiae and the anal, renal, and genital 

 orifices (Fig. 272, Br, a, r, g). In some 



FIG. 262. 



Internal shells of Cephalopoda. A, shell of 

 Belemnotvuthis dupiniana (Neocoiuian). /?, shell 

 of Sepvt Orbignyana. C, shell of Spirulirostra 

 Bellardii (Miocene) ; the specimen is cut so as to 

 show in section the chambered shell and the 

 laminated " guard " deposited upon its surface ; 

 A shell of Spinila laevis. (After Lankester.) 



Fio. 203. 



The chitinous internal 

 shell, orgladius.of Loligo, 

 the anterior ]rt upper- 

 most. (From Lankester, 

 after Owen.) 



forms it is divided longitudinally by a muscular junction between 

 the mantle and the visceral mass, starting from either side of 

 the anus : such is the case in the essentially littoral forms with 

 a short pallial sac, such as Sepiola and the Octopodidae. In 

 Opisthoteuthis the pallial cavity is particularly narrow and shallow, 

 and the pallial sac scarcely projects ; in consequence of the diminu- 

 tion of the ventral flexure the animal is flattened and discoid (Fig. 

 300), and the anus has returned to the posterior position which it 

 occupies in primitive Mollusca. 



Beneath the epithelium the integument contains, at least in 



